1. Field of the Description
This description is generally directed toward products such as polymer and other bank notes (or currency) with optical security features, and, more particularly, to a new configuration for an optical security element for products that provides high quality color imagery by addressing prior problems with registration during printing processes by using differing lens sets for each different, individual color to be printed in the ink layer (or image layer) positioned under a lens array in the optical security element.
2. Relevant Background
There are many products presently manufactured and distributed with optical security features so as to try to limit copying and counterfeiting. One of the most prevalent of these is currency of a country used daily in commerce. Other examples include tags or labels provided on clothing and other consumer items and credit and bank cards. It is desirable to provide optical security features to these and other products with minimal cost while also providing high levels of anti-counterfeiting protection. The anti-counterfeiting market is rapidly growing worldwide with anti-counterfeiting elements placed on a wide range of items such as upon currency (e.g., on a surface of a paper bill to help prevent copying) and on labels for retail products (e.g., labels on clothing showing authenticity).
With regard to protecting currency from copying, polymer bank notes or currency are made from a plastic or polymer such as biaxially oriented polypropylene (BOPP), blown propylene film, or the like. A growing number of countries are considering or even converting from paper to polymer bank notes, with at least eight countries having fully converted to polymer bank notes by 2014. Lower costs are one reason for this conversion as the polymer substrate or body of the bank note makes this currency more durable and longer lived. However, anti-counterfeiting is another key reason that many countries are converting to polymer bank notes.
Security features that are provided on paper can also be provided on polymer bank notes. Additionally, though, new security features that cannot be provided with paper currency can be provided with polymer bank notes because the substrate or body of the bank notes can be provided to be transparent (herein, “transparent” is intended to mean translucent to transparent to light). Hence, a transparent window may be provided that is used to display a security image that allows the bank note to be authenticated. An optical security feature may take the form of a lens or lens array (e.g., a lenticular lens array (linear lenses) or an array of round, hexagonal, aspherical, or other-shaped lenses) that is used to display an image printed on an opposite side of the transparent substrate (e.g., an interlaced image). The displayed or visible image may be a three dimensional (3D) image, an image that is animated with movement of the bank note (or with differing viewing angles), an image provided by a full volume pixel map or moiré pattern, and/or provide other optical effects available through the use of lenticular, diffraction, and other optical technologies.
With the use of such optical security features, polymer bank notes are very difficult to counterfeit as the optical security features cannot simply be copied using scanning, photocopying, and other techniques used with some paper bank notes. In many polymer bank notes, the security or anti-counterfeiting features are provided by a lens or lens array that is cast or embossed on the front or back of the bank note (or its transparent substrate or body) and by a corresponding image (e.g., a printed image visible through the lens or lens array, which may be considered the image element or component) provided on the reverse side of the bank note.
In any type of printed lens array (e.g., a lens array with linear, round, hex, aspheric, or other-shaped lenses paired with a printed ink or image layer) when multiple colors are used in the ink layer, all of the colors are printed under each lens. In other words, pixels (or printed dots) from an interlaced image used to create the print files or “plates” are provided under each lens of the lens array. FIG. 1 shows an exemplary optical security element 10 that includes a lenticular lens array 12 that focuses (as shown with arrows 13) light onto an ink layer 16 that is printed upon an opposite side 15 of a clear substrate 14 (or upon the back of the lens array 12 itself). In this conventional lenticular element 10, two or more colors are printed in the ink layer 16 underneath each lens, and this often leads to two colors being printed in the same position or with overlapping of the two (or more) colors of ink due to registration limitations of the printer or printing process. Hence, a viewer sometimes will see a lower quality color image with ghosting or other problems as the lenses 12 focus 13 upon two or more colors from viewing angles.
The registration requirements during printing increase dramatically as the pitch or frequency of the lenses increase. In other words, printing a linear lens at 75 LPI (lenses per inch) in a 4-color process is difficult but is far easier than a pixel-mapped round lens in multiple colors at 1,200 lenses per inch in two axes. The registration requirements for multiple colors under one lens can be as difficult as having a registration tolerance of less than 1 micron in two axes to get the proper frames in the proper colors back to the viewer. In web and sheet-fed printing processes, movements of 30 to 300 microns are common and often within specification of the manufacturer of the equipment.
Since in traditional lens technologies all of the colors in the image must be printed under each individual lens for the image to work properly to the viewer, these tolerances are very small. For instance, in a 75 LPI lens that has a focal length of about 18/1000-inch, a normal print resolution of about 2400 DPI (dots per inch) is used. The width of the lens is about 0.0133 inches. For a typical image to be printed, this is divided into about 8 segments or equal to 0.001666 inches per image frame. For this to work to the viewer for proper viewing, these image frames in a linear lens are generally printed in a 4-color process or can be individual colors (e.g., a CMYK color model used in color printing using four inks of cyan, magenta, yellow, and key (or black)). This means that each of the images must register to within about the amount of each image frame under each lens. While this can be done with traditional sheet-fed equipment, the dynamics get far more difficult as the lenses get smaller. Further, the registration requirements are not just in one axis, but, instead, they are in two axes (both X and Y or side to side and up and down).
One specific example is that a security thread for currency may have 1,200 lenses per inch or be about 21μ in diameter. In order to create a 3D or animated image, the number of image frames may be about 10 frames in two axes to achieve this. This equates to about a 2μ image frame. In order to make an image work and in multiple colors for a viewer, the registration requirements are about 0.5μ or less, which is impossible with any known processes.
Hence, there remains a need for new designs of optical security elements that make the registration requirements more forgiving, especially for thin security films and lenses that are not linear such as round, square, hexagonal and aspheric lenses used in many optical security elements.